Influence of lamp intensity and water transmittance on the UV disinfection of water

The efficiency of UV disinfection devices depends on flow, lamp intensity and water transmittance. In practice the flow is controlled by suitable methods, whereas the other two parameters, to date, have not been taken into consideration sufficiently. The surveillance of the function of UV disinfection plants is routinely based on a sensor placed on the irradiation chamber wall. This measures changes of UV irradiance without differentiating if the effect is caused by transmittance or lamp intensity so that two different conditions of disinfection may occur at the same sensor readings. We investigated the influence of transmittance vs intensity on disinfection at the same sensor readings in a specially designed laboratory flow through UV irradiation system with one single UV lamp as well as in commercially available UV disinfection plants with multiple UV lamps. All devices were equipped with a calibrated selective UV detector connected to a UV radiometer. Lamp intensity was decreased by diminishing the supply voltage. UV transmittance was reduced by pumping aqueous sodium thiosulphate solution into the water inflow. The disinfection capacity was determined by measuring the reduction equivalent doses (RED) using a standardised biodosimetric method. We showed that equal sensor readings, either achieved by reducing the lamp intensity or by lowering the UV transmittance of the water, resulted in different REDs in one-lamp systems. The diminishing of UV intensity caused a greater decrease of REDs than reducing the water transmittance. However, in the multiple-lamp systems tested, equal sensor readings yielded equal REDs.